The present invention relates to a camshaft phase changing drive and more particularly to a camshaft phase changing device for varying the timing of the valve actuation by an engine driven camshaft.
Laid-open Japanese utility model application No. 6-28203 published on Apr. 15, 1994 discloses a variable camshaft phaser employing a pair of axially spaced annular gears disposed and engaged between co-axial drive and driven members. The drive member is in the form of a sprocket having an internal helical spline, while the driven member is in the form of a stub shaft, having an external helical spline, secured via a front cover to a camshaft by a screw. The pair of axially spaced annular gears, a front or outer gear and a rear or inner gear, have inner and outer splines engaging the external and internal helical splines. The front and rear gears are biased toward one another for lash take-up by a plurality of angularly spaced gear pins press-fitted in the rear gear and having head compressing coil springs. In order to axially move the front and rear gears in one direction to vary the phase relationship between the sprocket and stub shaft secured to the camshaft, an annular piston is disposed adjacent the inside face of the rear gear and mounted thereto by a plurality of angularly spaced piston pins. The piston pins are fixedly secured to the annular piston and extend forwardly through a plurality of openings, respectively, of the rear gear toward, for abutting engagement with, the inside face of the front gear. Snap rings are mounted around these pins, respectively, to limit axial and rearward displacement of the annular piston away from the rear gear. The annular piston carries an outer peripheral seal in sealing contact with a finished cylindrical inner surface of the sprocket and an inner peripheral seal in sealing contact with the opposed finished cylindrical surface of the stub shaft. The annular piston and its outer and inner peripheral seals cooperate with the inner cylindrical surface of the sprocket and the outer cylindrical surface of the stub shaft to define an annular chamber. The annular piston may be axially movable in one direction in response to oil supplied under pressure to the annular chamber. The annular chamber is disposed on one side of the annular piston adjacent the rear gear. A coil return spring extends between the annular piston and a rear flange of the stub shaft to urge the annular piston in a return direction compressing the annular chamber, i.e., a direction opposite to the one direction. Owing to this return spring, the piston pins are urged to abut on the inside face of the front gear. Return movement of the annular piston is initiated by discharging oil under pressure from the annular chamber. The passing of the piston pins through the rear gear to extend into abutting engagement with the front gear has a benefit. During stroke of the annular piston in the one direction, the front gear is pulled behind the rear gear. During return stroke of the annular piston, the rear gear is pulled behind the front gear. Thus, during each stroke of the annular piston, the separation of the front and rear gears from one another is increased slightly, reducing the lash take-up force, thus reducing the friction that opposes motion of the front and rear gears.
Such arrangements is shown in copending U.S. patent application Ser. No. 08/406,302 filed on Mar. 17, 1995 by Seiji TSURUTA and commoly assigned herewith. This U.S. patent application has corresponding applications in Korea and Germany, i.e., Korean Patent Application No. 95-5704 filed on Mar. 18, 1995 and German Patent Application No. 195 09 845.5 filed on Mar. 17, 1995.
In manufacturing variable camshaft phasers of the above kind, a very careful attention must be paid in finishing component parts and assembling them to keep alignment of an annular piston relative to the adjacent rear gear of a pair of annular gears to avoid undesired friction that opposes motion of piston pins relative to the rear gear.
During operation of the variable camshaft phaser of the above kind, the stub shaft is subject to alternating torque from the camshaft and this alternating torque is translated by helical spline connection into alternating thrust, causing the pair of annular gears and the sprocket to vibrate. Various measures have been taken to cope with this problem. According to a known measure, an O ring seal is operatively disposed between the front cover and the sprocket to apply an axial bias force to the sprocket to reduce noise inducted by collision between the sprocket and the associated parts. This axial force creates the friction that opposes angular motion of the sprocket relative to the front cover and the stub shaft. The magnitude of axial bias force depends primarily on load vs. defelction characteristic of the O ring seal and a distance between the bottom of a seal groove receiving the O ring seal and the adjacent front end of a forwardly extending hub of the sprocket. As is well known, the O ring seal provides a large rate of increase in load against a unit increase in deflection over a predetermined range of deflection. Thus, a strict control of this distance is needed in manufacturing the variable camshaft phasers of the above kind.
An object of the present invention is to improve a variable camshaft phaser of the above kind such that, with less strict alignment control, no undesired friction that opposes the motion of piston pins relative to a pair of annular gears is created.